Proxied.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.6.0;

abstract contract Proxied {
    /// @notice to be used by initialisation / postUpgrade function so that only the proxy's admin can execute them
    /// It also allows these functions to be called inside a contructor
    /// even if the contract is meant to be used without proxy
    modifier proxied() {
        address proxyAdminAddress = _proxyAdmin();
        // With hardhat-deploy proxies
        // the proxyAdminAddress is zero only for the implementation contract
        // if the implementation contract want to be used as a standalone/immutable contract
        // it simply has to execute the `proxied` function
        // This ensure the proxyAdminAddress is never zero post deployment
        // And allow you to keep the same code for both proxied contract and immutable contract
        if (proxyAdminAddress == address(0)) {
            // ensure can not be called twice when used outside of proxy : no admin
            // solhint-disable-next-line security/no-inline-assembly
            assembly {
                sstore(
                    0xb53127684a568b3173ae13b9f8a6016e243e63b6e8ee1178d6a717850b5d6103,
                    0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF
                )
            }
        } else {
            require(msg.sender == proxyAdminAddress);
        }
        _;
    }

    modifier onlyProxyAdmin() {
        require(msg.sender == _proxyAdmin(), "NOT_AUTHORIZED");
        _;
    }

    function _proxyAdmin() internal view returns (address ownerAddress) {
        // solhint-disable-next-line security/no-inline-assembly
        assembly {
            ownerAddress := sload(0xb53127684a568b3173ae13b9f8a6016e243e63b6e8ee1178d6a717850b5d6103)
        }
    }
}